How to compare two files

I am inexpert with awk.
I tried with a K-shell to compare a  file against a sample file, record by
record and just a 16 char portion of each one, if both portions are equal
then I write out the full record to another file. It was very slow.
So my question, it's possible to do that in awk, but quite faster?.
I would appreciate any proposition. Thanks.

1) input file A  (size file A > 100000 records)
record A format (; field delimiter)  PLAS;SUB015678889F32442410R
;?;CCQ4567;?    ;6788797

^                                ^

|_______________|

key A

2)sample file B (size file B > 100000 records)
record B format (no field delimiter) CCU1001345789F46784544469
^
^

|_______________|

key B

3) IF key A .EQ. key B
THEN
PRINT RECORD A > OUT_FILE